1. Field of the Invention
The present invention generally relates to devices for testing the transmission quality of optical fibers, and more particularly to an optical time domain reflectometer having improved fault detection and location.
2. Descriotion of the Prior Art
In recent years, fiber optic cables have replaced traditional copper wire as the preferred medium for telecommunications Although optical fibers have certain advantages over copper wire, they are still subject to faults which may result during installation of the fibers or from environmental factors after installation. Also, the practical length of an optical fiber is limited by attenuation of the light signals travelling therein, since there can never be 100% transmission of light through these fibers.
Accordingly, it is necessary to occasionally test the transmission quality of optical fibers. One device which has established itself as one of the more versatile instruments for this purpose is the optical time domain reflectometer, commonly referred to as an "OTDR." In its simplest construction, an OTDR includes a light source, such as a pulsed laser diode; an optical coupler, including a beam-splitter, connecting the light source to the near end of the fiber under test (FUT); and a photodetector positioned adjacent the beam splitter. When a test signal is sent down the FUT, backscattering and reflections within the fiber core return to the near end of the FUT and are sensed by the photodetector. The trace signal of the backscattering and reflections provides clues as to faults in the FUT. Numerous of which are disclosed in the following patents and applications:
______________________________________ Patent/application Applicant ______________________________________ U.S. Pat. No. 3,981,592 D. Williams U.S. Pat. No. 4,070,118 Maslowski et al. U.S. Pat. No. 4,197,007 Costa et al. U.S. Pat. No. 4,212,537 Golob et al. U.S. Pat. No. 4,289,398 R. Robichaud U.S. Pat. No. 4,397,551 Bage et al. U.S. Pat. No. 4,497,575 H. Philipp U.S. Pat. No. 4,674,872 S. Wright U.S. Pat. No. 4,685,799 M. Brininstool U.S. Pat. No. 4,708,471 Beckmann et al. U.S. Pat. No. 4,732,469 M. Souma U.S. Pat. No. 4,743,753 Cheng et al. U.S. Pat. No. 4,838,690 Buckland et al. U.S. Pat. No. 4,870,269 Jeunhomme et al. Brit. Pat. No. 1,560,124 Standard Tel. & Cables Brit. Pat. Appn. 2,182,222 STC plc. ______________________________________
The backscattered signal (also known as Rayleigh scattering) is typically weak, and is due to refractive-index fluctuations and inhomogeneities in the fiber core. The strength of the backscattered signal is primarily dependent on the peak power and width of the test pulse, i.e., a longer pulse width results in stronger backscattering. The backscattered signal may be used to detect faults such as micro-bends or splice losses, and to measure overall attenuation. In fact, attenuation is primarily due to backscattering, although it is also a function of the wavelength of the test pulse and any discrete losses along the fiber path.
Reflective signals (also known as Fresnel reflections) are somewhat stronger, and are due to discontinuities in the fiber. The strength of the reflected signal is primarily dependent upon the peak power of the test pulse. Reflective signals may be used to determine the overall length of the fiber line, and to detect breaks in the fiber, reflective connectors, and splices of fibers having different indices of refraction. Reflective signals also cause "deadzones," as explained more fully below.
Although the trace signal is a function of time (i.e., the amount of time passing from the initial test pulse until the return signal is detected), it can be directly correlated to positions along the FUT by the equation x=ct/2n, where x is the distance along the fiber, c is the speed of light in a vacuum, t is the elapsed time, and n is the index of refraction of the fiber material. Thus, the approximate location of a fault or splice may be determined.
A difficulty arises in locating faults, however, due to the deadzone created by Fresnel reflections. If two faults are in close proximity, their reflections and/or losses will overlap and may appear in the trace signal as a single fault. The theoretical length 1 of the deadzone is 1=ct.sub.pw /2n, where t.sub.pw is the duration of the pulse width. For example, an OTDR emitting a 500 nanosecond pulse into an optical fiber having an index of refraction of 1.5 will result in a deadzone of about 50 meters, which is quite significant. Of course, other factors can exacerbate this effect, such as the response time of the photodetector, and the strength of any reflected signals.
In order to minimize the deadzone and thereby increase the effective resolution, a small pulse width may be selected. Prior art OTDR's provide for manual selection of pulse width from a set of a few discrete values. Some OTDR's provide a pulse width as small as one nanosecond. In minimizing the deadzone, however, other performance parameters of the OTDR are adversely affected. As noted above, micro-bends and splice losses are detected by means of Rayleigh scattering which is dependent on the pulse width. Hence, if relatively small pulse widths are employed, low loss microbends and splices may go undetected, although they would be distinguishable if the launch signal were longer. Attenuation in the fiber may make it difficult to detect distant faults, further mandating a longer pulse width. More broadly stated, a single trace may provide an optimal pulse width for one section of the fiber path, but the pulse width will not be optimal for the majority of the path. This presents a clear dilemma which prior art OTDR's have not adequately addressed.
The above problem relates only to the resolution of the OTDR for purposes of detecting the fault. Another problem occurs with respect to the precision of the OTDR in determining the location of any given fault along the fiber path. Early OTDR's merely provided a graphic display of the return trace signal from which only the crudest estimates could be made. Instruments have since been devised which can automatically detect and toggle through the approximate locations of losses, but they still require heavy user interpretation with respect to the specific location of any given fault.
For example, some prior art OTDR's employ digital sampling and analysis of the trace signal, and use a moving least-squares fit of several datapoints to calculate an average slope function. Logic circuitry examines this function for deviations which are greater than a preset threshold value, and records the elapsed time (i.e., the distance along the fiber) to the datapoint corresponding to the change in slope. The calculated distance, however, is usually not the actual distance to the fault. In order to more accurately define the specific point at which the fault occurs, human interaction is necessary. These prior art OTDR's allow the user to graphically estimate the fault location by moving a cursor on the display to the point along the trace signal corresponding to the beginning of the fault. This is, of course, a very subjective step and requires experience and training for an reliable measurement. It is clear that a simpler and more accurate technique for fault location is long overdue.
It would, therefore, be desirable and advantageous to devise an optical time domain reflectometer providing optimization of pulse width, and improved resolution in fault detection and location. It should also be capable of multi-fault operation, and should calculate the loss value at the fault. Finally, minimal operator training and interaction should be required.